Re: How and Why do cats purr?? How do dogs hear better than humans??

I'm afraid I don't know why cats purr, and it seems from my brief internet 
search that no-one else does either.  Try entering 'cat purr' or somesuch 
in a search engine, and you will find plenty of discussion of the 
subject.  One of the pages which come up is a Madsci.com answer -  http://madsci.wustl.edu/posts/archives/may96/828639116.Zo.r.html

As to dogs hearing better than humans, I am not sure that they do.  We are 
often surprised by the abilities of animals just because they differ from 
our own, and are are therefore outside our experience.  Dogs - and cats - 
can, for instance, hear higher frequency sounds than we can, and there is 
recent evidence that big cats may communicate using very low frequency 
sound.  I suppose that if their hearing covers a wider range of frequency  
than our own, their hearing could be said to be 'better' on the grounds 
that bigger = better.  

Dogs hear a wider range of frequencies than we do, and possibly have more 
sensitive hearing than humans because they are adapted to a different way 
of life where it important to be able to hear quiet, high pitched sounds.  
Through evolution, dogs could be said to have 'invested' more design 
effort and materials in their hearing.  These investments could 
include 'hardware' such as bigger ears, more accurately designed parts 
within the ear, more nerves from the ear and additional brain space to 
analyse sound.  They could also include 'software' such as more elaborate 
sound processing power within the brain.

This raises the question as to why we should have poorer quality hearing.  
If it is possible to hear as well as a dog, why don't all mammals have 
this ability ?  Has our hearing been 'dumbed down' for some reason ?  

Yes and no.  It's no better than it ought to be.  To understand this 
problem, you have to put yourself in the position of an evolving animal 
species and do a cost/benefit analysis of each part of your body.  
Remember that all living things (except modern humans) live on the edge of 
survival.  They normally have only just enough resources to stay alive and 
do the things they need to do.  Their food and energy must be carefully 
invested in exactly the right ears, legs, nose, etc for the job they have 
to do.  Surplus performance equals wasted resources, and a threat to 
survival.

Sure, it would be great to have the best of everything, but if you had 
claws like a lion, ears like a bat and arms like a gibbon you might find 
that they were not actually an advantage in carrying out the normal 
activities of a human - and you would look pretty strange as well !